Preheat oven to 350°F (175°C). Line an 8x8-inch square baking pan with parchment paper or lightly grease it.
Mix dry ingredients: In a large bowl, stir together the rolled oats, cinnamon, baking powder, and salt.
Combine wet ingredients: In a separate bowl, whisk the egg, maple syrup, almond butter, vanilla extract, and milk until smooth.
Combine mixtures: Pour the wet ingredients into the dry ingredients and stir until just combined.
Fold in apples: Add the diced apples and any optional add-ins like walnuts or raisins. Stir gently to distribute evenly.
Transfer to pan: Pour the mixture into the prepared baking pan and spread it into an even layer.
Bake: Bake for 25–30 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean.
Cool and slice: Let cool in the pan for 10–15 minutes. Then transfer to a wire rack and cool completely before slicing into 9 bars.